Technical Field
The present disclosure relates to a micro-electro-mechanical device with compensation of errors due to disturbance forces, such as quadrature components.
Description of the Related Art
As is known, MEMSs (Micro-Electro-Mechanical Systems) are used in an increasingly widespread way in different applications, due to their small dimensions, costs compatible with consumer applications, and their increasing reliability. In particular, with this technology inertial sensors are manufactured, such as microintegrated gyroscopes and electro-mechanical oscillators.
MEMSs of this type are generally based upon micro-electro-mechanical structures comprising a supporting body and at least one mobile mass coupled to the supporting body through springs or “flexures”. The springs are configured for enabling the mobile mass to oscillate with respect to the supporting body according to one or more degrees of freedom. The mobile mass is capacitively coupled to a plurality of fixed electrodes on the supporting body, thus forming variable capacitance capacitors. The movement of the mobile mass with respect to the fixed electrodes on the supporting body, for example under the action of external forces, modifies the capacitance of the capacitors; thus, it is possible to detect the displacement of the mobile mass with respect to the supporting body and the external force. Instead, when suitable biasing voltages are supplied, for example through a separate set of driving electrodes, the mobile mass may be subjected to an electrostatic force that causes movement thereof.
To obtain micro-electro-mechanical oscillators, the frequency response of the MEMS structures is usually exploited, which is of a second-order low-pass type, and has a resonance frequency.
MEMS gyroscopes, in particular, have a complex electro-mechanical structure, which typically comprises at least two masses that are mobile with respect to the supporting body, coupled to each other so as to have a number of degrees of freedom depending upon the architecture of the system. In the majority of cases, each mobile mass has one or two degrees of freedom. The mobile masses are capacitively coupled to the supporting body through fixed and mobile sensing and driving electrodes.
In an implementation with two mobile masses, a first mobile mass is dedicated to driving and is kept in oscillation at the resonance frequency at a controlled oscillation amplitude. The second mobile mass is driven with oscillatory (translational or rotational) motion and, in case of rotation of the microstructure about a gyroscope axis at an angular velocity, is subjected to a Coriolis force proportional to the angular velocity itself. In practice, the second (driven) mobile mass acts as an accelerometer that enables detection of the Coriolis force and detection of the angular velocity. In another implementation, a single suspended mass is coupled to the supporting body to be mobile with respect to the latter with two independent degrees of freedom, and precisely one driving degree of freedom and one sensing degree of freedom. The latter may include a movement of the mobile mass in the plane (“in-plane movement”) or perpendicular thereto (“out-of-plane movement”). A driving device keeps the suspended mass in controlled oscillation according to one of the two degrees of freedom. The suspended mass moves on the basis of the other degree of freedom in response to rotation of the supporting body about a sensing axis, because of the Coriolis force.
As has been mentioned, to enable the MEMS gyroscope to operate properly, a driving force is applied that keeps the suspended mass in oscillation at the resonance frequency. A reading device then detects the displacements of the suspended mass. These displacements represent the Coriolis force and the angular velocity and may be detected using electrical reading signals correlated to variations of the capacitance between the second (driven) mass and the fixed electrodes.
However, MEMS gyroscopes have a complex structure and frequently have non-ideal electro-mechanical interactions between the suspended mass and the supporting body. Consequently, the useful signal components are mixed with spurious components, which do not contribute to the measurement of the angular velocity. The spurious components may depend upon various causes. For instance, manufacturing defects and process spread are potentially inevitable sources of noise, the effect whereof is unforeseeable.
A common defect depends upon the fact that the oscillation direction of the driving mass does not perfectly matches the degrees of freedom desired in the design stage. This defect is normally due to imperfections in the elastic connections between the suspended mass and the supporting body and causes onset of a force directed along the detection degree of freedom of the angular velocity. This force in turn generates an error, referred to as “quadrature error”, due to a signal component of unknown amplitude, at the same frequency as the carrier and with a phase shift of 90°.
In some cases, the quadrature components are so large that they may not simply be neglected without introducing significant errors. Normally, at the end of the manufacturing process, calibration factors are used in order to reduce the errors within acceptable margins. However, in many cases, the problem is not completely solved, since the amplitude of the quadrature oscillations may vary during the life of the device. In particular, the supporting body may be subject to deformations due to mechanical stresses or temperature variations. In turn, the deformations of the supporting body may cause unforeseeable variations in the movements of the masses and, consequently, in the quadrature components, which are no longer effectively compensated.